drm/i915: Define some of the eLLC magic

The EDRAM present register isn't really defined in the docs. It just
says check to see if it's set to 1. So I haven't defined the 1 value not
knowing what it actually means.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>